In many high temperature applications, it is necessary to join together tubular parts of substantially different chemical and physical characteristics. For example, such applications arise in fossil-fired boiler construction and in nuclear power stations. In such power stations, high temperature joints are commonly required in various heat exchanger components such as boilers, steam generators, intermediate heat exchangers and recuperators, particularly in high temperature gas-cooled reactors, etc. Similar applications arise in other industries, such as petrochemical equipment and chemical processing plants having substantial requirements for heat exchangers, steam lines and the like.
In all of these applications, it is usually necessary to form large numbers of tubular interconnections between materials of substantially different characteristics. Usually, one component of the tubular interconnection is formed from a high temperature alloy that is particularly suited for withstanding high temperature environments. At the same time, it is necessary for the interconnection or transition joint to withstand similar severe operating conditions of temperature, pressure, etc., over extended periods of time.
Under conditions of the type described above, the different types of materials to be interconnected by the transition joint exhibit substantially different physical characteristics making it difficult to maintain continuity throughout the transition joint. For example, the existence of very different thermal expansion rates on opposite sides of any given bond within such a high temperature joint tends to produce particularly severe stresses, tending to cause cracking or total disruption of the bond. Other factors also exist within such transition joints which further interfere with the maintenance of an effective continuous transition joint or interconnection.
In many such high temperature applications, the different tubular materials to be interconnected consist of a low alloy steel or carbon steel on the one hand and, on the other hand, a high temperature alloy composition adapted to best withstand the severe high temperature conditions. Welding of such steel components is particularly contemplated by the present invention, and the high temperature alloy may contain approximately 16-20% chromium or more. Examples of such high temperature alloys include wrought or cast austenitic steels, particularly austenitic 300 series stainless steels, such as ASTM or ASME type 321H stainless steel or 304H stainless steel. The low alloy, carbon-containing steels may be ASTM or ASME SA213 Grade T-22 steel containing, for example 21/4% chromium, 1% molybdenum.
Substantial efforts have been expended in the past to develop effective transition joints for such applications. One such approach has been the formation of the joint with continuously changing chemical composition along the length of the joint, for example, by electroslag techniques, with one end of the joint being joined to the other tubular piece of substantially different composition and characteristics. Many different types of materials, such as powder metallurgy components and the like, have also been employed to form transition joints. Heat treatment has also been employed both prior to and following formation of the transition joint in order to better condition the transition joint and interconnecting bonds to withstand severe operating conditions of the type referred to above.
A recent solution to the problem of dissimilar metal weldments (DMWs) has been the use of stepped transition joints. In such joints, a member is pre-formed from a plurality of sections that are welded together. The sections of the member are selected to generally provide a gradient of chemical composition and physical properties between the austenitic and ferritic components which are being joined together. For example, the chromium contents and thermal coefficients of expansion of the several segments are generally progressively graded between one end of the member and the other. While such stepped transition joints have generally been found to work well, multiple welded members are relatively expensive.
There remains a need for improved and simpler techniques for welding dissimilar metals, particularly for welding austenitic components to ferritic components.